Some technologies improve tamper-resistance by making it difficult for any cracker to probe a signal flowing in a circuit board. For example, Patent Literature 1 discloses a method for judging whether or not an unauthorized access by probing has been performed, by outputting a detection signal to a component that is to receive a confidential signal, and detecting a waveform distortion in the detection signal received by the component.
The following describes the technology disclosed in Patent Literature 1 with reference to FIGS. 30 and 31.
FIG. 30 is a cross-sectional view of a circuit board 1000.
When a confidential signal flows from a terminal 1116 of a component 1111 to a terminal 1117 of a component 1112 (by way of a foil 1121, a via hole 1122, a foil 1123, a via hole 1126, and a foil 1127), the component 1111 outputs a detection signal, and the component 1112 receives the detection signal and checks whether or not the waveform of the received signal is normal.
FIGS. 31A through 31C illustrate examples of waveforms of the detection signal flowing between the component 1111 and the component 1112.
FIG. 31A illustrates a normal waveform of the detection signal. On the other hand, in FIGS. 31B and 31C, the solid lines indicate abnormal waveforms of the detection signal that are observed when an observation point 1113 or the like is probed.
According to Patent Literature 1, an abnormal waveform with more gradual rise or fall as illustrated in FIG. 31B is detected by comparing a period of normal reception which is stored in advance, with a period in which the potential exceeds a high reference potential 1211 or a period in which the potential falls below a low reference potential 1212. Also, an abnormal waveform with ringing occurring in the rise or fall as illustrated in FIG. 31C is detected by judging whether or not the potential exceeds a high reference potential 1221 or judging whether or not the potential falls below a low reference potential 1222.